Computing devices, such as notebook computers, personal data assistants (PDAs), mobile communication devices, portable entertainment devices (such as handheld video game devices, multimedia players, and the like) and set-top-boxes (such as digital cable boxes, digital video disc (DVD) players, and the like) have user interface devices, which are also known as human interface devices (HID), that facilitate interaction between the user and the computing device. One type of user interface device that has become more common is a touch-sensing device that operates by way of capacitance sensing. A touch-sensing device usually is in the form of a touch-sensor pad, a touch-sensor slider, or touch-sensor buttons, and includes an array of one or more capacitive sense elements (e.g., a capacitive sense array). The capacitance detected by one or more capacitance sensors changes as a function of the proximity of a touch object to the touch sensor. The touch object can be, for example, a stylus or a user's finger.
One type of capacitance sensing device includes multiple touch sense electrodes arranged in rows and columns and forming an array of intersections. At each intersection of the electrodes in the X and Y axes (i.e., a location where the approximately orthogonal electrodes cross over, but do not connect with, one another), a mutual capacitance is formed between the electrodes thus forming a matrix of capacitive sense elements. This mutual capacitance is measured by a processing system and a change in capacitance (e.g., due to the proximity or movement of a touch object) can be detected. In a touch-sensing device, a change in capacitance of each sense element in the X and Y axes of the touch sense array can be measured by a variety of methods. Regardless of the method, usually an electrical signal representative of the capacitance of the capacitive sense elements is measured and processed by a processing device, which in turn produces electrical or optical signals representative of the position of one or more touch objects in relation to the touch-sensor pad in the X and Y axes. A touch-sensor strip, slider, or button may operate on the same or another capacitance-sensing principle.
A touch-sensing device may be coupled to and/or affected by one or more sources of noise, such as an external power supply coupled to the touch-sensing device. These sources of noise may produce noise in the touch-sensing device when a touch object comes into contact with the touch-sensing device and the touch object completes a circuit connection between the noise source (e.g., an external power supply) and the touch-sensing device. The noise caused by the contact between a touch object and the touch-sensing device may be referred to as direct coupled noise. The direct coupled noise caused by the contact between a touch object and touch-sensing device may affect the capacitance measured by processing device. This may affect the ability of the processing device to detect contacts between the touch object and the touch-sensing device.